This invention relates to the transmission and reception of digital signals along subscriber telephone loops and is partially concerned with mitigating the adverse effects of the digital signals interfering with amateur radio transmissions and vice versa.
In an access system for use in a fiber-to-the-neighbourhood (FTTN) network, digital data is exchanged between a host digital terminal (HDT) and a plurality of optical network units (ONUs) via optical fibers. Each ONU is responsible for exchanging downstream (ONU-to-subscriber) and upstream (subscriber-to-ONU) data with a respective plurality of subscribers via copper twisted pairs. Typically, the downstream and upstream data are modulated about separate carrier frequencies and occupy spectral bands which have a width proportional to the transmitted data rate. For example, a data stream at 20 Megabits per second, modulated using 16-QAM (quadrature amplitude modulation with 16 constellation points, i.e., 4 bits per symbol), can be transported by a spectral band having a width of 5 MHz and being centered about a given carrier frequency.
Two major factors affecting the performance of a system which transmits data along a twisted pair are loop attenuation and crosstalk. In order to combat these two elements, it is commonplace to use a lower-order modulation format, such as 16-QAM, and to restrict the frequency range of the upstream and downstream data to approximately between 1 MHz and 20 MHz.
Unfortunately, this frequency range is also subject to interference from amateur radio communications carried by HAM bands commonly known as the 160-, 80-, 40-, 30-, 20-, 17- and 15-meter bands, whose spectral characteristics are illustrated in the following table:
Clearly, a downstream or upstream spectrum of any considerable width (i.e., data rate) will straddle at least one HAM band. By way of example, FIG. 1 shows a signal spectrum 11 having a center frequency fC of approximately 9.0 Mz and a bandwidth fS of approximately 5 MHz. The signal spectrum 11 is seen to span two HAM bands, specifically bands 14 and 13 (centered at 7.150 MHz and 10.125 MHz, respectively).
Due to various regulations governing electromagnetic interference, it is imperative that a signal transmitted along the twisted pair contain reduced signal energy in any HAM band so as not to interfere with HAM radio communications occurring in those bands. This requirement is somewhat in conflict with the goal of transmitting high data rates, and has forced telecommunications companies to provide ways of reducing or eliminating the signal energy in these bands, while still delivering the high data rates demanded by today""s customers,
In one prior art approach, the total downstream (or upstream) data rate is handled by transporting the data using multiple separate carriers, which allows placement of the individual spectra between HAM bands, thereby avoiding any intersections of the signal spectra with HAM bands. However, this technique increases the complexity of the transmitter and receiver, as both must now be equipped to deal with multiple parallel modulations or demodulations.
Another prior art approach acknowledges that the transmitted signal spectrum will straddle one or more HAM bands, and the signal to be transmitted is passed through a aeries of digital notch filters in the transmitter, each filter notch being centered about a HAM band. These filters may operate at passband or at baseband. If a passband filter (or series of filters) is used, then the sharpness required of the filter (or filters) is very high, necessitating the use of a large number of taps. On the other hand, if baseband filters are employed, the notches arc generally asymmetrically disposed about zero frequency, requiring the use of complex coefficients. Either solution leads to a relatively high complexity for the transmitter.
Even if notching at the transmitter were able to prevent the signal spectrum from interfering with the HAM bands, it cannot control the onset of interference due to transmissions occurring in the HAM bands themselves. That is to say, the receiver will necessarily accept a signal whose spectrum has undergone radio-frequency interference due to HAM transmissions. In order to combat this effect, a conventional approach applies band-pass and notch filtering at the receiver in order to pass only those components of the signal spectrum lying outside the HAM bands. Again, regardless of whether filtering is performed at passband or at baseband, the complexity of the required filtered is relatively high.
Finally, the art has seen the development of a technique known as DMT (discrete multi-tone), which relies on the transmission of a large number of carriers generated by inverse Fourier transform techniques, each carrier being associated with a small amount of the overall required bandwidth. Radio-frequency interference is sidestepped in the DMT system simply by employing only those carriers which do not overlap with the HAM bands. However, the processing requirements of an actual implementation of DMT are often too demanding to permit cost-effective use of this technique.
It is an object of the present invention to mitigate or obviate one or more disadvantages of the prior art.
Therefore, the invention may be summarized according to a first broad aspect as a transmitter for transmitting a modulated signal across a transmission medium, the transmitter comprising: an encoder for encoding a digital data stream into one or more encoded digital signals; one or more substantially identical baseband notching filters connected to the encoder, for respectively receiving the one or more encoded digital signals, each baseband notching filter having a notch at zero frequency; and a modulator connected to the one or more baseband notching filters, for producing the modulated signal centered about a carrier frequency, wherein the carrier frequency is approximately equal to the center frequency of an interference band.
According to a second broad aspect, the present invention may be summarized as a receiver for extracting a digital data stream from a modulated signal, said modulated signal being centered about a carrier frequency, the receiver comprising: adaptive interference cancellation (AIC) means for controllably reducing narrowband interference present in the modulated signal around the carrier frequency, thereby to produce an interference-reduced signal; a demodulator connected to the AIC means for receiving and demodulating the interference-reduced signal, thereby to produce one or more baseband demodulated signals; a power estimator connected to the demodulator and to the AIC means, for receiving the one or more baseband demodulated signals, calculating the power of residual interference present in the one or more baseband demodulated signals around the carrier frequency and providing the AIC means with an interference estimate signal; one or more substantially identical baseband notching filters connected to the demodulator, for receiving the one or more baseband demodulated signals and producing respective filtered demodulated signals, each baseband notching filter having a notch at zero frequency; and a decision-feedback equalizer (DFE) connected to the one or more baseband notching filters, for receiving the filtered demodulated signals, decoding digital data embedded therein and producing the digital data stream.
The invention can be summarized according to another broad aspect as a method of transmitting digital data, comprising the steps of: encoding the data into one or more baseband digital signals; filtering the one or more baseband digital signals with respective baseband filters having notches at zero frequency; modulating the filtered baseband digital signals about a carrier frequency, thereby to produce a modulated signal; and transmitting the modulated signal across a transmission medium; wherein the carrier frequency is approximately equal to the center frequency of an interference band.
According to yet another broad aspect, the present invention may be summarized as a method of recovering a digital data stream from a received signal modulated about a carrier frequency, comprising: demodulating the modulated signal, thereby to produce one or more baseband demodulated signals; filtering the one or more baseband demodulated signals with respective baseband filters having notches at zero frequency; and decoding the filtered baseband demodulated signals, thereby to recover the digital data; wherein the carrier frequency is approximately equal to the center frequency of an interference band.
According to still another broad aspect, the present invention may be summarized as a modem for transmitting a first modulated signal onto a twisted pair and for receiving a second modulated signal from the twisted pair, comprising: a hybrid for interfacing with the twisted pair; a transmitter connected to the hybrid, for producing the first modulated signal from a first digital data stream, the transmitter comprising an encoder for encoding the first digital data stream into one or more encoded digital signals; one or more substantially identical baseband notching filters connected to the encoder, for respectively receiving the one or more encoded digital signals, each baseband notching filter having a notch at zero frequency; and a modulator connected to the one or more baseband notching filters, for producing the first modulated signal centered about a carrier frequency, wherein the carrier frequency is approximately equal to the center frequency of an interference band; and a receiver connected to the hybrid, for extracting a digital data stream from the second modulated signal, the receiver comprising adaptive interference cancellation (AIC) means for controllably reducing narrowband interference present in the second modulated signal around the carrier frequency, thereby to produce an interference-reduced signal; a demodulator connected to the AIC means for receiving and demodulating the interference-reduced signal, thereby to produce one or more baseband demodulated signals; a power estimator connected to the demodulator and to the AIC means, for receiving the one or more baseband demodulated signals and providing the AIC means with an estimate of residual interference in the one or more baseband demodulated signals around the carrier frequency; one or more substantially identical baseband notching filters connected to tile demodulator, for receiving the one or more baseband demodulated signals and producing respective filtered demodulated signals, each baseband notching filter having a notch at zero frequency; and a decision-feedback equalizer (DFE) connected to the one or more baseband notching filters, for receiving the filtered demodulated signals, decoding digital data embedded therein and producing the second digital data stream.
The present invention may be summarized according to still another broad aspect as a method of allocating a frequency spectrum, comprising; selecting a first portion of the frequency spectrum for transmission of a first signal, said first portion having a first center frequency fC1; wherein fC1 is related to the center frequency fH1 of an interference band by fC1 being approximately equal to fH1.
According to a further broad aspect, the present invention may be summarized as a method of allocating a frequency spectrum, comprising: selecting a first portion of the frequency spectrum for transmission of a first signal, said first portion having a center frequency fC1 and a bandwidth fS1; wherein fS1 and fC1 are related to the center frequencies fH1 and fH2 of two interference bands by fC1 being approximately equal to xc2xd(fH1+fH2) and fS1 being approximately equal to 2|fH2xe2x88x92fH1|.